1. Field of the Invention
The present invention relates to a cationic electrodeposition coating composition with excellent throwing power, suitability for electrodeposition onto hot dip galvanized steel sheets, finished appearance and corrosion resistance.
2. Description of the Related Art
As conventional cationic coating compositions containing blocked polyisocyanates there may be mentioned cationic electrodeposition coating compositions comprising base resins with cationic groups such as amino, ammonium, phosphonium and sulfonium or such cationic groups that have been neutralized, and blocked polyisocyanates as crosslinking agents (curing agents).
Examples of blocking agents used for blocked polyisocyanate compounds for cationic electrodeposition coatings include alcohol-based, ether alcohol-based and oxime-based blocking agents, which are used to take advantage of their coated film corrosion resistance and coating stability.
One conventional blocked polyisocyanate that has been disclosed has an aromatic isocyanurate ring and three or more blocked isocyanate groups, with at least one of the blocked isocyanate groups being blocked by a glycol ether (Japanese Unexamined Patent Publication No. 7-233238).
There has also been disclosed an invention which is a cationic electrodeposition coating composition comprising a block isocyanate curing agent and an epoxy-modified base resin with a cationic group, where the blocking agent for the block isocyanate curing agent employs a propyleneglycol monoalkyl ether (Japanese Unexamined Patent Publication No. 2001-192611).
There has further been disclosed a cationic coating composition containing a cationic resin (A) and a blocked polyisocyanate wherein a polyisocyanate compound is blocked with a blocking agent comprising a diol component with a molecular weight of 76-150 having two hydroxyl groups with different reactivities (Japanese Unexamined Patent Publication No. 2002-241690).
However, when high voltage is applied to the blocked polyisocyanate-containing cationic coating compositions disclosed in the aforementioned patent documents in order to achieve high throwing power, the result is an insufficiency in either the inhibition of pinhole generation on hot dip galvanized steel sheets or in the finished appearance or corrosion resistance.